The liquid crystal display (“LCD”) has already been a leader of information display devices and its market occupation ratio is very high. It has advantages of low power consumption and light weight. But it has drawbacks of narrow viewing angles and low speed response in comparison with the cathode ray tube (“CRT”). These drawbacks have not yet been resolved. Among these drawbacks of the LCD, the issue of narrow viewing angles is a particularly fatal problem as the size of the display panel is becoming larger and larger. To solve this problem of narrow viewing angles, the following methods have been introduced.
First, there is the in-plane switching (“IPS”) method for driving the liquid crystal by forming electrodes on only one side of the liquid crystal substrate to improve the characteristics of the viewing angles. This method is the one utilizing the configuration of the electrodes. According to this method, the dark state is obtained if no electric field is applied under the condition that the liquid crystal is aligned in parallel to the substrate and the optic axis of the liquid crystal is coincident with one of the polarizers (one of which is perpendicular to the other). The bright state is obtained when the electric field is applied. According to this method, wide viewing angles may be obtained because the change in the average optic axis occurs on a plane that is parallel to the surface of substrate. However, the response time becomes longer than the response time of the LCD in twisted nematic (TN) mode because twisted distortions occurs mostly. Furthermore, the aperture ratio is low in this method because the electrodes are formed on only one side of the substrate. In addition, the production yield is also low because the default in relation to the alignment occurs frequently.
Second, there is a method to obtain wide viewing angles utilizing an alignment structure of the liquid crystal. For example, the structure of an optically self-compensating Pi (π) cell has been introduced. In this structure, the thickness of a liquid crystal cell is determined so that there would be a phase lag of a half wavelength from the light projected vertically to the substrate when the initial director of the liquid crystal is maintained. The liquid crystal is aligned to form an angle of 45° between the direction of liquid crystal molecules projected on the substrate and the optic axis of the polarizer. In this way, the double refraction of the tilted incident light becomes less than that of the vertically incident light. Therefore, the viewing angles are made relatively wider in the off state. However, this method has the difficulty of controlling the alignment properties precisely.
Another example is a method utilizing the reverse TN effect, i.e., the homeotropic to twisted-planar (HTP) transition effect, by applying voltage in an initial state of the vertical alignment. In this method, a chiral dopant and a nematic liquid crystal which has negative dielectric anisotropy are used. According to this method, a high transmissivity is obtained because of the driving mechanism which is reciprocal to that for the general TN. A perfect dark state can be obtained between the two perpendicular polarizers at the off state. Thus, high contrast ratio and superior viewing angles may be obtained in comparison to the general TN. However, symmetric viewing angles still cannot be obtained in the on state.
Third, there is a commonly used method of utilizing an optical compensation film. In this method, the change in double refraction dependent upon the azimuthal angle is compensated by an uniaxial optical compensation film because the TN-LCD has a structurally asymmetric property at the time of driving. However, this method has problems because color dispersion is caused by the wavelength-dependent refraction dispersion of the liquid crystal and because the manufacturing process is complex and the production cost is high.
In addition to the above-mentioned methods, several methods which improve the viewing angle characteristics by inducing the change of the optic axis in different domains of a unit pixel using the multi-domain (MD) alignment are known. For example, the MD-TN (Multi-Domain twisted nematic) method obtains the symmetrical viewing angle dependent upon the azimuthal angle by dividing each pixel into four multi-domains and by causing the direction of the distortion of the nematic liquid crystal to be different in each domain. However, this method also has a shortcoming because the manufacturing process is complex since different rubbing processes of different directions are required for multi-domains and, thus, the production yield is low and the production cost is high. Furthermore, the reproducibility is low because of the defects that arise on a boundary of each domain during the driving.
As a similar method to the MD-TN, the MD-VA (multi-domain vertical alignment) method is known. In this method, the initial aligning direction is maintained to be vertical in each domain of a unit pixel. Thus, the light leakage is very low in the off state and the contrast ratio is high. However, this method has also problems, as with the MD-TN method, because each domain must go through the rubbing process of a different direction for each of the domains. The manufacturing process is complex because several alignment processes should be applied. Furthermore, defects may occur on a boundary of each domain during the driving.
Another method, the a-TN (amorphous twisted-nematic) method, is also known. It is a technique for improving the viewing angles by forming very small domains of arbitrary aligning directions in a unit pixel without rubbing processes. This method has an advantage in that the manufacturing process is very simple. However, it is practically impossible to control sizes of the small domains because the small domains are formed in arbitrary sizes. Further, the reproducibility is low.
The recently proposed ASM (axially symmetric aligned microcell) method obtains the circularly symmetric viewing angles by mixing the liquid crystal with polymer and using the phase separation in each pixel. In this method, it is possible to obtain uniform alignment of the liquid crystal on a large area without rubbing processes. However, there is a problem regarding the reliability of polymer. Furthermore, the precise control of the phase separation is difficult and the manufacturing process is complex. Thus, this method cannot be applied to mass production of LCDs.